A serial cat killer in south London may be keeping the heads and tails of victims as "trophies", according to a criminologist.

As many as 50 animals - mostly cats - are thought to have been mutilated by the so-called Cat Ripper of Croydon over a period of three years, although the number of cases has risen dramatically in recent months.

There are concerns the killer is keeping the heads and tails so they can "re-live the moment" of slaughter, and might one day attack humans.

The Boston Strangler Albert DeSalvo, Milwaukee Cannibal Jeffrey Dahmer, and the students behind the Columbine High School massacre were all cruel to animals prior to attacking humans.

Criminologist Dr Adam Lynes from Birmingham City University who specialises in serial murder told Sky News: "There are some parallels with serial killers who target humans who take trophies such as watches, underwear, jewellery, or even hair.

"And it could be the case that this person is keeping the heads and tails (of the cats) as trophies, to re-live the moment.

"Given that they dump the cats' bodies at - or near - the homes of the owners would suggest they like the attention."

Image Caption:Ukiyo was found dead on a neighbour's doorstep in Croydon in 2015

Wayne Bryant, who lives near Croydon, found the remains of his tortoiseshell cat Amber in woodland behind his home a day after she went missing.

He said: "My wife found her body. Her head was missing, her tail was missing. We found out afterwards that she also had a broken back. That's quite a level of violence against a small, helpless animal.

"The way she was laid out - she wasn't just thrown. She was put there to be found in that position.

"I'm sure she was put there like that to have more of a shock factor because it wasn't instantly clear the head was missing. It was only after you picked her up that you realised there was no head."

The majority of the mutilated animals have been found in south London, but there have been other cases in Stepney Green, Charlton, and Finchley Central.

There is no indication that the net is closing in on the killer - or killers.

A reward of £5,000 has been offered by animal rights activists desperate to find the killer.

Kirsty Henderson from People for the Ethical Treatment of Animals said: "The majority of these animals have been cats but unfortunately other species have also been killed and mutilated.

"We have seen reports of foxes who have been mutilated - killed of course - and left on the street. So who knows where this person will stop?

"We don't want to frighten people in this community, but make them aware of what is happening and to ask them to keep their companion animals inside, keep them safe, be diligent, look out for what is happening in your community and hopefully together we can catch this person."

:: Anyone with information is urged to contact police on 101 or call Crimestoppers anonymously.